Unsustainable Curiosity
by EmpressKira
Summary: Marco couldn't help his curiosity when coming across a siren that almost dragged him to his death. The crew is worried he will be driven too far with curiosity and it seems they were right when they wake up to find him gone. Made for MerMay. MarcoAce (barely). Rating T to be safe.


**A/N:: Here is for MerMay! Yes, it's basically the last days of the month! But better late than never! I would like to thank my friend on tumblr, fire-lark, for helping with the plot and giving motivation to continue it since i had only a small part!**

 **Hope you all enjoy~!**

* * *

 **Unsustainable Curiosity**

Ships battles could range from anywhere being a large scale or a simple canon to destroy the other. A bell resounding through the noon air, sun high in the sky to bring a humid heat among anyone lingering. The hollers and soon resounded cannon fire is almost a daily occurrence for above the Maiden's berth. Pirates, marines, traders, and simple fisherman could be caught in a battle if not careful of where they sail through. On the glistening clear blue waters is that of two pirate ships; hollers and resounding banter echoing as swords clashed and pistols fired.

 _ **! ! ! !**_

Water splashing fiercely and only a few noticed as a man with blond hair had fallen overboard. No one had a moment to step and retrieve him, not knowing if he were wounded or merely in the watery depths now and being able to swim. Shouts of someone to follow was there, calling that their _Commander_ was in need of them to retrieve him if he may be injured.

In the waters, the blond with a designed bandana in his hair was floating along and trying to steer clear of dropping cannon balls. Blood colored the water from his thigh wound he received and had been the cause of his stumble before a rifle butted to his cheek to get him over the railing. Choice words entered through his mind as he shifted around to soon face the depths below with his body, but was looking around to his sides to see of any predators.

"Mmmm~hhhmmm…" A soft melody reached his ears and blue eyes searched for the source of it. The sound drew closer with it switching to low humming before the pirate situated to be facing down and was startled at the sight before him.

Dark eyes gleamed up at him with a low sound leaving the throat as the Adam's apple moved for it. Freckles sprinkled the male's pale skin, dark hair lingering in a caress to the water and his finned ears had shifted. They were fanned out, spots of sorts cluttering them and as the pirate skimmed more to notice that of a toned torso, he could see the gills being along his ribs. It was unclear what his tail looked to be, but the blond man didn't seem to pay much attention as a hand with webbing between the fingers raised towards his face.

Lips parted from the creature, "Aaa~aaahhh~…" The sound clouded the pirate's mind almost instantly, mouth lightly parting as he felt himself drawn in. It was as if the melody was telling him of promises so enduring to his ears, it couldn't be helped as he wanted more of those dreams he sought. Answers to everything he sought in the world, a man who was drawn to knowledge with high _curiosity_ and sought adventure along with it _._

Dark eyes shined dangerously as nails lengthened and the other webbed hand joined to slowly touch along the pirate's face carefully so. The sudden hand grasping to the pirate's clothing, jerking the blond back, had the creature hiss in anger and move hands to grip to the human's neck that he was claiming as his own. It was of another pirate, and the sudden movement and change of tone woke the blond from the trance. A struggle began as the creature began hugging onto the pirate in a firm hold in his arms around the head as the strong tail of the water's beast hit the auburn-haired companion of the pirate. In a quick turn, the creature began swimming down to the depths with resistance to his captives struggles. Though, he wasn't expecting when a dagger stabbed into his side, a screech left the creature and released the pirate while quickly swimming away.

The pirate kept his dagger prepared and was soon kicking to begin swimming up, trying to hold his breath. Lungs strained under such restraint and the pressure of the ocean was heavy on his chest. Blue eyes found the crew mate that waited with a hand out, amber eyes meeting him in worry and fear of what had just happened. Making it to the other in time, the blond was helped to the surface where his companion hollered for a rope and _quick_.

Back on the ship, the crew waited with bated breath on what had them shaken up. "It was a _beast_!" Amber eyes were still wide as he ran fingers through his auburn hair and the one he helped save was next to him with wide eyes as well as he was trying to calm his racing heart.

"That was a _siren_ , Thatch." The blond explained, already knowing of the legends that every sailor knew and even sung of.

"You met a maiden of the sea? I thought you had always wanted that?" One asked curiously before both snapped eyes in their direction.

"That was a male and he was about to drag Marco to the _bottom_ of the _ocean_!" Thatch exclaimed before trying to calm himself, adrenaline still racing through him and others were shocked at the two who are in top strongest of the crew.

"A siren? What did he promise?" It was a sincere question, going off legends and myths of the beings.

"I don't… I don't remember, yoi…" Marco mumbled lightly with a glance back towards the ocean, curiosity flaring through him on why creatures like sirens dragged their prey to the depths. The crew noticed the blond showing that side where he would do something dangerous if not watched for a while, just until the stemming curiosity was overruled by logic.

They couldn't lose their Commander.

 **-o-o-o-o-**

The crew were highly aware of their Commanders curiosity brimming, blue eyes showing to be casted over the shimmering waters on more occasions than not. Thatch tried his best to keep the man distracted enough, mentioning of promising their Captain how they would find the island with the treasure they are to receive. Their crew had a home base where they all could rest and meet up, also switch men around if necessary. The crew hardly wanted to go after this treasure with how Marco was reacting with this past encounter that should have left the blond traumatized; but instead left the Commander _curious_.

That's why when the crew woke up one morning to find him gone—they panicked.

 **-o-o-o-o-**

Marco made sure he pulled his small dinghy onto the shore, his shoes being of sandals as he didn't want anyone to hear him in his boots when leaving the boat. He had been lucky to make it away from the rest of the crew, only thinking of how he would return after delving more into what reasoning the creature had. If anything in this world that Marco wanted would be of knowledge and to quench his curiosity. The Maiden's berth is large and deep, carrying so many histories and along with that he gazed upon the Divine's lands when they found each island.

Though he would not explore this island as he let himself pull off his shirt and bandana to rest to the boat with his shoes and the small bag he brought along. It carried notebooks and feathers with ink so he may write later if he got the chance. Walking barefoot, he waded through the waters to calf deep first as he looked over the dark berth as night only carried that of a half-moon in the sky. A strain pulled at his chest, thinking when he used his dagger had scared the creature completely to other waters. In a way, he hoped that the siren had lingered near the ship, maybe to show itself again.

"Mmmmmhhaaa~…" The soft melody immediately had the pirate shifting towards the ocean more as it grew soft again. Feet moved along the sand, letting it stir as he moved carelessly to about waist deep as the melody had only calmed down back to silence. Marco flickered his gaze to the water, heart racing as he craved to see the Maiden's creature with his own eyes again. Lips shifted to speak, but he could only really make a shaky breath as he continued to peer for any sign. Nothing followed, having the pirate sigh out before he felt skimming along his calves. "Aaa~laa~…" The melody immediately had him in a calm before he was pulled into the waters in a harsh yank. Water pressed along his skin as he was quickly pulled to the deeper waters, but Marco didn't resist. The blond merely tried to stay calm as he was soon released and the creature swam around in a flurry of reds and pale yellows.

The creature gazed upon the human as he noticed no resistance, feeling a bit more calmed at that reaction since he wouldn't have to trance him. Webbed hands shifted to touch along the blond's chest as the pirate did not wear a shirt this time, only in his shorts. It was innocent curiosity and he only lingered momentarily as he swam around before touching along legs with the feel of his smooth hands. The human carefully moved a hand to his mouth to hold in his breath, the siren noticing immediately and moved before the pirate that only viewed him in similar interest. Dark eyes flickered to soon touch along ribs, noticing as no gills were there like his and soon remembered of the Divine's creatures before swimming off as the witch told him to retrieve the thing they needed.

Panic filled Marco as the creature quickly darted off towards the ocean floor, and it had him turn to see if a predator was around. Seeing nothing was good, but he soon had to begin swimming to the top as his lungs constricted for the need to breathe. As he began kicking for the surface, a hand grasped his ankle and started pulling him back down. The last of his air left him, facing defeat on the fact he wouldn't reach to the surface and soon turn his head to notice as a hand gripped to his chin. Fingers pressed to his lips and he could merely open them to feel something inserted into his mouth. It felt slimy and basically _dreadful_ , but he swallowed as he notices dark eyes looking to him expectantly.

As the creature pulled away, the pirate jerked with hands to his throat and tried to get the strange sensation away. Coughs left him, water actually being _blocked_ from his airway as he soon noticed skin splitting and stretching under his fingers. Shock flourished over his features as he soon noticed that he was _breathing_ and moved hands to feel along the gills formed along his neck. They flared as he breathed in before relaxing when exhaling and his blue eyes searched the creature before him. A smile started to form on the creature as he noticed the gills and the man breathing before latching around to his waist.

"I won't be alone anymore!" The sudden voice had startled Marco as he noticed it to be the creature and couldn't move from the grip. "The witch was right! You can stay beneath the waters~!" A melody followed the last word in a beautiful rhythm of pure joy and soon the creature shifted to swim around the pirate.

"What?" Marco took the moment to speak, figuring out after a few noises that he could—though it sounded rather muffled.

"I'm Ace, and you are my companion!" The creature had stopped to float in the water as a pure look was upon him and showed of hands together by his chest. "We can travel together! See more of the ocean! The other oceans I have heard of!" A joy played along the creatures face and the pirate was confused as he soon moved hands out to grab the others attention.

"You can't just make people your companions, yoi…" The blond mentions while he noticed the body stiffen with the head turning so dark eyes looked to him in confusion.

"What? Why not?" It was utter confusion on the freckled face as finned ears flared out a little and eyes narrowed. "I-I picked you! I don't just grab anyone! You seek what I do as well!" Ace showed hands out in waves of his explanation and nails were sharpened with his irritation.

"But I have a crew! A ship where people considered as my brothers would worry of me!" The blond came back with that and soon noticed trembles from the creature as he began to notice the tail having cracks, glowing brightly in the emotional creature. "I'm not going to abandon you, Ace." The pirate spoke quickly, noticing the similar sheen in eyes that showed of a possible trance going to leave the siren. "I just… I am trying to understand, yoi…" The words were chosen carefully as he showed himself to be open, having Ace look to him curiously before approaching to be before him with eyes staring up to him.

"The witch…" The siren began as he slowly moved to be back somewhat and seemed nervous in his spot. "She told me that if I had the _Shrillweed_ with me that I could help a human breath under water. A compatible being that wouldn't want me for other means that could harm me and so, I just…" There was pause as he gazed towards the dark depths and gave a noise of slight restraint. "I just thought, trancing a human to be calm and taking them to it would be faster… But… but the few before you kept… kept _dying_ when getting close to the weed." Ace explained in sincere perplexity and was showing out hands, the webbing stretching to thin a bit. "I knew they still had air, but then they would suddenly blow it all out and then just… _die_ and I… I didn't _mean_ for them to die…" There was a strain within the creature's words as lips trembled with a soft melody of hurt leaving.

"We can't survive the water's pressure towards the bottom…" Marco spoke out lightly in explanation before noticing eyes looking to him in confusion.

"Because of the pressure? But it's only a little bit and it takes a few seconds to merely adjust." The creature spoke out, immediately moving closer as the pirate kept himself open to show he wouldn't do anything irrational.

"It's the same with if we pulled you above water, even if I gave you something to breath… taking you to even the smallest mountains would cause severe pressure as you are not used to them." The explanation had made Ace tilt his head in thought before seeming to realize on where the human was getting at before a horrified look dawned the freckled face.

"I… I have been… leading them to their death…" The realization was agonizing to watch and Marco tried to coax the other. "I thought… I was helping…"

"Ace, it's okay… you didn't know, yoi." Hands carefully moved to touch the creature along the shoulders, skin feeling a bit of a tiled texture but also smooth with the water. Dark eyes found blue ones as the creature moved to press along the human with his tail curling around a side and angled for the flaring tail fin to be away from the human. Arms clung to a neck, but careful not to block gills as Ace trembled with a clenched jaw.

"I just… I just don't want to be alone anymore…" The desperate tone had Marco cringe while soon grabbing a hold of the other and soon closed eyes as his face pressed to the side of the other's head.

 _He didn't want to leave Ace alone…_

 **-o-o-o-o-**

"WHY DIDN'T YOU TELL ANYONE?!" The practical screech had anyone nearby in the early hours of the morning cringe away with gritted teeth. The sun hadn't even made the sky of a pink yet when the man with long dark hair back in a braid began yelling. The clothes fit loosely on his frame and his slender figure still held muscles that would knock you out for thinking or saying anything wrong, being the perfect crew advisor.

"Now, Izo," began the auburn-haired man who waved hands nervously and tried to be more awake for such a thing. It was disconcerting that someone let their Commander slip away in the night, but it couldn't be helped. Plus, the ship had already begun in that direction when the night watchman was brought up, Izo immediately getting others to hoist sails and redirect their course. "Let's just head off, that small island isn't far and we should prepare for the monster anyways."

A sigh lingered as the dark-haired man rubbed to his neck, "you are right. We need to go retrieve our stupid Commander…" It was of a mutter as no one wanted to bring up the possibility of their fellow brother being possibly dead—dragged to his death.

They plotted what to do, going in numbers instead of only a couple of them to overrun the beast of the sea. They knew only what of Thatch told them of the creature and to not be fooled as it turned ravenous as soon as it felt threatened. Thoughts of what have become of their Commander ran through everyone's head, whether he was left at the bottom for dead or did the creature feast upon humans? It seemed all farfetched, but then again it was farfetched on beautiful women with soft voices would cradle them to the depths. Not possible beasts, being male or female, dragging them with a horrid appearance. The fact that they even existed was troubling and they tried to keep the negatives away as nothing was good about their brother dying.

As expected, the island could not be ported, but that didn't stop them all from getting onto the few small boats left to get closer to the island as one was sighted. They had a feeling it was Marco's and soon started preparing with more plausible weapons for underwater; mostly being harpoons or giant hooks with ropes. As they all began to sail along closer, they noticed their boats rocking and they all heard the softly humming begin, giving them the sign that the creature was below.

"Get it! But don't kill it! We have to try and figure out if it took Marco to his death or just… holding onto him somewhere!" Izo tried to stay positive and gripped the short harpoon with a tight grip as others agreed. "Into the water!" The raised voice was there before they all moved to jump out of the boat and into the watery depths as the sun was completely above the horizon.

 **-o-o-o-o-**

"Stay here," the pirate was confused on the sudden change of mood as Ace had been leading him around to show what he could without going too deep. At some point, the siren tried to even give a raw fish to Marco, who mentioned how he wasn't sure how that works with the shrillweed and didn't know if the fish was dangerous to him. Ace was learning more of how humans were and things that were a bit not okay for them, though in turn the pirate learned more of the other.

Being left in a small cove, not too far from where he had come up to shore in his dinghy, he adjusted around as things sounded odd. Of course, he lingered in the water, not sure if it was okay to get out, but a ringing was in his ears as he soon took a chance. Crawling out of the water, his ears rang louder and he sputtered lightly before there was a pop. A wince left him as warmth was in his ears and he noticed that his throat didn't feel as blocked as it was before. Sounds were clearer and he heard noises outside of the cove of splashing, but nothing more. Marco could only think there was a predator of sorts that Ace was protecting him from.

Marco learned that the creature merely sought someone to be with, travelling for so long by himself. When asked about the witch, he said that he happened to come across her on a small island in a shack right above the waters and a small walkway to the island. It hadn't been his plan to stop by there, but he was pulled in to find himself talking to the woman who spoke sweetly and told of what he wanted. Ace mentioned it had been funny that him, a siren, was drawn in by someone and couldn't help but heed her words.

Of course, then the creature would grin as he called Marco his first companion. Those words would only cause the pirate's heart to race and appreciate the innocence of the other. Ace just didn't want to be lonely, someone fearing him all the time that he was bad and would trance them. It took Marco a minute, but seeing how the creature was proved enough on how innocent Ace was of his thoughts. Keeping the person calm to soon show them the world beneath the waves, quenching the desire to explore and learn new discoveries. All Ace asked in return was for them to not abandon him, as he wished so badly to be above the waves, but knew of no way to do so.

Hollers sounded out, startling the Commander as they sounded familiar and he immediately moved to get up. Making his way along the sand barefoot, a hand pulling at his belt to be tighter for his damp clothes, he noticed around a boulder that his crewmates were along the beach now. The first recognizable with braided hair and loose, damp clothing. Moving closer with furrowed eyebrows, the first man to notice was Izo and began grinning with a wave out.

"There you are, Marco!" The man quickly ran over as it wasn't too far, but just enough to run for it. As Izo approached, Marco glanced to see the others at the water trying to pull something out. "We were worried when we didn't notice you with the beast." The words came out, making the Commander realize on _who_ they were dragging out of the water.

"Y-You can't bring him on land, yoi…" The tone was said with a shaky breath of shock as all he noticed was of Ace being tied with ropes, hooks within his body to be the anchors to keep the ropes in place.

"Marco, we are not letting it kill you!" Izo protested while trying to snap the other's attention back, but noticed the panic flourishing through blue eyes.

"You are going to kill him!" The words were spoken harshly while the blond quickly moved in a dash across the beach and through the shallow waters. Others began to stop him, pulling at him to stay away from the creature that was beginning to gasp in the air as the water was becoming too shallow for him. "Get him out of the ropes!" Marco hollered as he tried to reach the siren that was finally pulled enough to have just an arm in water, barely reaching his gills on one side. Most pulled away from the being that thrashed and tried to figure out what was going on with their Commander.

"A-Aaa~lahaaa~…" The melody began, making most to cover their ears, but the others holding back Marco or just merely gazing in shock noticed it was a different tone than what they have been told. It was sorrowful, _broken_ as if the creature was realizing that he could possibly die where he lays.

Marco would not stand by so easily as he was able to break through and run over to the creature with his heart racing, chest constricting in fear. Seeing the skin looking to already be dry and lips parted with trembles. It was evident Ace was experiencing asphyxiation, almost as if he was drowning in the open air. Partially the blond noticed quivering gills as the creature tried to breath in his means of air and was hardly getting any. The sight has the pirate shaking before desperately trying to pull the rope to get Ace free, but the ropes were sturdy as he should know as they have caught sharks and whales with them. Everyone stood shocked as he continued to pull before making contact with dark eyes that dulled as the melody lowered.

The look alone had Marco soon gripping tightly and began to quickly pull the creature towards the water. Nothing else ran through his head except to get Ace into the water to _breathe_ , and even his brothers moving to try and stop him did not deter his motive. When reaching far enough in the water, the creature thrashed to prevent the other humans from stopping his drag to freedom. As they got far enough where the blond was to his chest in water, the creature harshly moved his body against the other to press him under the water with him.

Both disappearing under the waves.

 **-o-o-o-o-**

Being back in the ocean had Ace heal quickly enough, but as they got the ropes from him, Marco moved a hand to show to wait. Anxiety flared through the siren as the blond swam back to the shore line, making him want to follow, but wasn't sure. Seeing his companion go along closer to shore only made everything flare in him with anxiety, but was only quelled as he could still see bare feet lingering with water going up to almost mid-calf.

The creature paced, fearing that his companion really would leave him in the end. It was true that Marco had a family on the ship and at the island he spoke of, but Ace had no one. The only one he has is the pirate, not even the witch would make a good companion, that's part of why he sought one out. It was evident that Marco liked him, the clarity was blaring obvious as he could sense what someone craved. Curiosity had been there when Marco decided to come find him, though Ace followed the ship in determination and merely waited for a good time to grab the blond for himself.

As he paced, it didn't take long before the pirate began to come back to deeper waters, elating Ace as he restlessly waited. Though, when the other stopped about waist deep, he eyed the body posture before the man dunked his head and waved a hand for him to come closer. Nothing gave him a bad vibe, but Ace didn't trust the other land dwellers, so he took it cautiously. Moving closer to his human that raised back up once closer, he slowly brought his head above the water to give quick glances, but could not see anyone near, but then again, his view was blocked.

"It's easier," Marco began with a reassuring smile on his face, "for me to speak like this, do you mind?" The creature relaxed at the soothing tone as it showed to make sure to his comfort zones were considered. Once Ace came a little closer, Marco gave a hand out so he could grip onto it and feel a steady place to anchor him down. "I told them that you would not harm anyone and that they _will not_ harm you either. You are important to me, yoi." Blue eyes flickered as a smile was there and the creature began to grin as he moved closer to grip around the waist of the human, who chortled with a hand through his damp hair as his face rest to an abdomen. "To make you more comfortable, I convinced Thatch to come along to speak with you in the water. Do you think you can get more of that stuff you used on me to be able to linger under the water?" The question came out before dark eyes peered to the Commander and a nod was there before moving to dive back under the water.

 **-o-o-o-o-**

After receiving the shrillweed, Marco took a few more minutes to convince his brother to take it before popping it into his mouth. Thatch followed suit and was soon underwater with flails of the weird sensations he gained before noticing the blond begin to swim forward. The crewmate followed as he noticed the creature, still making him nervous, but was startled as the siren grasped a hold of his brother. The sound of laughter was there while Thatch noticed that the creature was hugging the blond, who was grinning with light protests.

As the crewmate got closer, the creature looked unsure and gripped to Marco tighter while the pirate shifted to view his brother. "This is Ace. Ace, this is Thatch." The introduction came out while the siren examined him thoroughly with dark eyes before seeming to brave coming closer.

It made Thatch nervous, but his brother promised that the creature would not hurt him and from the interaction made him somewhat at ease. Not only that, but he trusted Marco to know that he meant his words, so he let the siren approach. Hands slowly moved to reach and skimming fingers along him as he stayed stiff, not wanting to make a wrong move. Ace continued his light touches, swirling around to touch on his legs, careful to not startle him as he gave nervous jerks. Once the creature was done and back to in front of him, there was a curious flicker in eyes.

"Do… Do you want to be my companion too?" It was so innocently asked that Thatch was a bit surprised before realizing what their Commander had mentioned. The creature was lonely and did not know what he was doing was killing the humans as he just wanted to calmly talk with them. It was unorthodox, yes, but Thatch could see as the siren was unaware of this spectrum.

"I… I don't see a problem with that…?" It was a slight question before arms were around his upper body as he was shifted around by a wiggling body.

"T-Two companions! I have two! I can't be lonely now! I have two!" This joy pierced the water with a melody of pure happiness lingering and Thatch looked to his brother who smiled, making him fully begin to understand more.

 _Ace just wanted someone there with him_.

"Sorry about what we did…" Thatch commented while carefully patting to a shoulder and the creature pulled back with pursed lips, but soon fought through with a smile.

"I… I am too. I didn't want you to take Marco from me." Ace replied as said blond swam up beside the two, letting the creature hug onto him. It was funny to see as wiggles left the siren, making Marco complain lightly as they were shifting around. "If I could, I would be on land with the land dwellers, but I do not know of any of that." The creature admitted while holding the blond tightly and wiggled more as Marco smiled to the antics.

"Ah! But there is a way!" Thatch claimed with arms crossing over his chest, but one shifted to have a hand stroking his goatee, feeling odd to do in the water. "Or so I have been told…" It was a considering thought, but Ace was immediately in front of him with gleaming eyes and hands to his chest.

"There is?!" Noticing the excitement brimming, the auburn-haired male quickly gave a nervous noise.

"Well, it was a legend of sorts." The words were to not get the creature too riled up.

"But Ace is of legend as well, yoi." Marco mentions as if pointing out a fact and soon gave a curious noise. "I don't think Oyaji would mind us trailing off on an adventure after returning the treasure home."

"If we can even make it there," the scoff left Thatch as the siren moved to watch the interaction and soon grinned.

"I can help! Let me help!" Ace spoke excitedly as he would very much like to pursue a chance to be of land and have _legs_ like his two companions have. The two pirates looked to one another, the blond already settled on helping the creature as best as he could and only made his brother sigh in exasperation.

"Alright, alright, I will look it up again in my books." A cheer left Ace as he quickly began swimming around the two with twirls added in, making them laugh. "For now, we should tell the others and possibly introduce you?" The last part is of questioning as he didn't want to pressure the siren, who had stopped and gave an uneasy look. "It will take some time, but they will get used to you." Thatch merely mentions as an added part to hopefully help pacify the fear stemming from what just happened.

"I… I can try…" A mentioning was there as Ace moved over to the blond as it was obvious he was the anchor to help steady him. "I would like to have more companions!"

"Ace, you can call us friends, it's alright…" Marco mentions in a coaxing tone and got a grin while gripping to the arm in his hands.

"Friends! I would like to have more friends!" The cheer left the creature as he wiggled more in happiness and the Commander began smiling happily at the outcome of it all.

After getting the others to see the good of Ace, they would journey to find the piece that will turn Ace to be among humans.


End file.
